Research On Protective Coating For Blade Tip Of Aero-engine

With the improvement of thrust-to-weight ratio and fuel efficiency,advanced aero-engines need to have superior gas-path sealing performance.Therefore,it is desirable to maintain a small radial clearance between the rotor blade tip and the corresponding seals to reduce the loss of the compressed gas during operation.Besides,due to the extremely high cutting speed(the relative linear speed can reach hundreds of meters per second)and instantaneous temperature rise(the highest temperature is nearly 1000℃)in the service process,the serious wear of blades tip and the high incidence of titanium fire were increasingly prominent.Furthermore,with the inlet temperature and gas pressure of engine increasing,the service environment at the turbine blades tip was drastically deteriorating,and the problems,i.e.wear,oxidation and corrosion,also need to be solved urgently.Currently,a new type of protective coating for blades tip with a unique design concept was an effective way to improve such problems.For the blades of titanium alloys,Ni/cBN and Ni/Si3N4 coatings were prepared on the blades tip of the titanium alloys by composite electroplating,and their high-temperature oxidation resistance,burn resistance and high-speed rubbing performance were studied,respectively.As for the blades tip of the superalloy,a NiCrAlYSi+NiAl/cBN coating with composite layered structure was prepared by the combination of electroplating,arc ion plating and chemical vapor aluminizing,and its high-temperature oxidation properties,hot corrosion performance and high-speed rubbing performance were investigated,respectively.In the paper,the degradation and failure models of these coatings in the corresponding tests were established to reveal the mechanisms.The results of combustion tests shown that the combustion of TC25 alloy was a liquid phase reaction,which can be divided into initial oxidation stage,ignition stage and stable combustion stage.It is clearly observed that three different districts were formed in the reaction area after combustion,viz.,oxide zone,melting zone and heat affected zone from outside to inside.An enrichmen t zone of refractory elements occurred at the interface between the melting zone and the heat-affected zone,which was considered to have a strong effect on reducing migration rate of solid-liquid interface.Oxidation tests indicated that Ni/cBN and Ni/Si3N4 protective coatings have superior oxidation resistance at 700℃.The flame retardant test results shown that two kinds of protective coatings can significantly improve the ignition point of titanium alloys.This is mainly due to the unique structure of the two coatings,which makes it difficult for dripping titanium droplets to contact directly with the coating and substrate,and the coating can effectively inhibit the inward diffusion of oxygen into the substrate,thereby playing the role of burn resistance and thermal barrier performance.The burn resistance and thermal barrier performanc of Ni/Si3N4 coating was better than that of Ni/cBN coating,which was mainly related to the properties of the two ceramic particles.During the rubbing process.the transfer layer aggravated the wear of the abradable coating because its higher hardness and it increased the blade length.However,the Ni/cBN and Ni/Si3N4 abrasive coatings can inhibit the materials transfer of Al/hBN coating,thereby greatly reduced the coverage and thickness of the transfer layers,and effectively extended the life of the blades tip.The failure mechanism of Ni/cBN coating was that cBN particles were covered by the transfer materials of abradable coating,and partly of them were pulled out during rubbing,while the failure mechanism of Ni/Si3N4 coating was that the protrusion parts of Si3N4 particles fractured during rubbing.Moreover,the abradability of Al/hBN coatings was remarkably improved when rubbed against abrasive coatings compared with bare blade.The high temperature oxidation and hot corrosion results shown that the abrasive coating has superior oxidation resistance at 900℃ but lost weight seriously at 1000℃.The coating forms a transient metastable 0-Al2O3 firstly,and then transform to α-Al2O3 during the isothermal oxidation at 900℃.Meanwhile,the parabolic rate constant(kp)would decrease by an order of magnitude as the oxidation products change.NaCl highly corroded the abrasive coatings by the cyclic chlorination/oxidation reaction.Besides,as a rapid transit channel of the corrosive media,the interface between cBN and the metallic coating also acted a pivotal part in hot corrosion.The presence of NaCl and the cBN/metallic coating interface triggered simultaneous damage mechanisms,which further accelerated the corrosion process.During cyclic oxidation,a nano-scaled interfacial product,namely AIN,was formed at the cBN/metallic matrix during aluminization,while the AIN transformed into a multilayer mixture of Al2O3 and 9Al2O3·2B2O3,resulting in a decrease in the bonding strength of the interface.Furthermore,the TCP phase in SRZ increased significantly,and the phase transition occurred.As a result,the stability of matrix structure and the strengthening effect of elements decrease sharply.The results of bending tests shown that the as-deposited abrasive coating-substrate system provided a superior bending toughness compared to the uncoated specimen,while that of the coated specimen after oxidation decreased sharply due to the severe elemental interdiffusion between the coating and substrate.The fracture mechanisms were as follows:for the coated specimen without oxidation,the cracks nucleated at the cBN/metallic matrix interface during bending tests,and the vertical cracks transited to horizontal cracks at the NiCrAlYSi coating/IDZ interface,eventually inducing the spallation of the coating.However,for the coated specimen with oxidation,the vertical cracks propagated into the substrate and some spread transversely along the IDZ and SRZ,eventually leading to the fracture of the substrate.This mainly depended on the degree of interdiffusion between the coating and the substrate before and after the cyclic oxidation,as well as the composition and microstructure of IDZ and SRZ.